Charge transport and recombination in heterostructure organic light emitting transistors (original) (raw)
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Charge transport processes in organic light-emitting devices
Synthetic Metals, 2000
The luminous efficiency of organic light-emitting diodes depends on the recombination probability of electrons injected at the cathode and holes at the anode. We have developed a numerical model to calculate the recombination profile in single-and multilayer structures, taking into account the built-in electric field, the charge injection process at each electrode, hopping transport with field-dependent mobilities, charge diffusion, trapping and Langevin recombination. By comparison of the simulation results, as well as approximate analytic solutions, with experimental data on MEH-PPV-based devices, we find that injection is thermionic with Schottky barriers for some electrode metals that are low enough to be considered Ohmic. Except at voltages close to threshold, diffusion and trapping effects are negligible. Both electrons and holes are mobile, with a field dependence that is independently confirmed both by single-carrier space-charge-limited current measurements and transient time-of-flight techniques. q 2000 Elsevier Science S.A. All rights reserved.
2006
We have realized a light-emitting organic field-effect transistor (LEOFET). Excitons are generated at the interface of an n-type and a p-type organic semiconductor heterostructure inside the transistor channel. The dimensions and the position of the p-n heterostructure are defined by photolithography. The recombination region is several microns from the metal electrodes. Therefore, the exciton quenching probability in this device is reduced. Numerical simulations show that the recombination region can move within the transistor channel by changing the biasing conditions.
Applied Physics Letters, 2006
We have realized a light-emitting organic field-effect transistor (LEOFET). Excitons are generated at the interface of an n-type and a p-type organic semiconductor heterostructure inside the transistor channel. The dimensions and the position of the p-n heterostructure are defined by photolithography. The recombination region is several microns from the metal electrodes. Therefore, the exciton quenching probability in this device is reduced. Numerical simulations show that the recombination region can move within the transistor channel by changing the biasing conditions.
We present experimental evidence of combined effects of temperature, carrier concentration, electric field as well as disorder on charge transport in an organic field-effect transistor (OFET). Transfer characteristics of an OFET based on sexithiophene active layer were measured from 80 to 300 K. Thermally activated carrier mobility followed Arrhenius law with two activation energies. Carrier density variation led to finite extrapolated Meyer-Neldel (MN) temperature (780 K) at low fields. Negative electric field-dependent mobility was observed in available field range. MN temperature shifted towards higher temperature when the electric field increased, and did not retain its finite character above the field of 4 × 10 3 V cm −1 .
Organic Electronics, 2011
Organic light-emitting transistors can be operated by an alternating gate voltage to provide high light output intensity. We propose a model for the light generation process in such light-emitting transistors based on systematic measurements of how the light output intensity depends on the biasing parameters. Following injection of holes, which form a positively charged space-charge region, subsequent electron tunneling from the same metal electrode eventually leads to light emission from the organic emitter. The electron injection is found to depend on the positive space-charge region, and the hole injection efficiency therefore strongly influences the emission intensity. Low temperature measurements show increased emission intensity as the temperature decreases indicating that the light generation is dependent on thermally activated charge injection, i.e., Fowler-Nordheim tunneling theory is not applicable. Further metal/semiconductor interface modification could result in optimized charge injection under AC biasing, thus leading to more efficient organic light-emitting devices.
Organic Light-Emitting Diodes with Field-Effect-Assisted Electron Transport Based on textlessitext
2008
Materials commonly used in the carrier transport layers of organic light-emitting diodes, where transport occurs through the bulk, are in general very different from materials used in organic field-effect transistors, where transport takes place in a very thin accumulation channel. In this paper, the use of a high-performance electron-conducting field-effect transistor material, diperfluorohexyl-substituted quaterthiophene (DFH-4T), as the electron-transporting material in an organic light-emitting diode structure is investigated. The organic light-emitting diode has an electron accumulation layer in DFH-4T at the organic hetero-interface with the host of the light-emitting layer, tris(8-hydroxyquinoline) aluminum (Alq 3 ). This electron accumulation layer is used to transport electrons and inject them into the active emissive host-guest layer. By optimizing the growth conditions of DFH-4T for electron transport at the organic hetero-interface, high electron current densities of 750 A cm À2 are achieved in this innovative light-emitting structure.
Injection, transport, and recombination in organic light-emitting diodes
Organic Light-Emitting Materials and Devices II, 1998
Efficient conversion of electrical to optical energy in organic light-emitting diodes (OLEDs) depends on balancing the flux of holes injected at the anode with that of electrons at the cathode. In this paper, we discuss several concepts related to optimizing the power efficiency of OLEDs, and put them in the context of analytic and numerical models for OLED operation. A simple argument is used to relate the charge injection rate from each electrode to measurable properties of the organic layer, deriving the equivalent of the Richardson-Dushman equation for the metal-organic interface. We discuss the role of charge density in dictating the importance of both space charge effects and recombination. These ideas are illustrated with experimental data from device structures which exemplify the various types of behavior predicted.
Light-emitting ambipolar organic heterostructure field-effect transistor
Synthetic Metals, 2004
We have investigated ambipolar charge injection and transport in organic field-effect transistors (OFETs) as prerequisites for a lightemitting organic field-effect transistor (LEOFET). OFETs containing a single material as active layer generally function either as a p-or an n-channel device. Therefore, ambipolar device operation over a wide range of operating voltages is difficult to realize. Here, we present a highly asymmetric heterostructure OFET architecture using the hole transport material pentacene and the electron transport material N,Nditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C 13 H 27 ). Efficient charge carrier injection is achieved by using Au as bottom contact for hole injection into pentacene and Mg as top contact for electron injection into PTCDI-C 13 H 27 . The device characteristic of this asymmetric heterostructure shows all features of ambipolar operation. For example, a typical transistor characteristic with a linear and saturation region is observed for small drain-source voltage V DS . For large V DS , the current increases due to additional injection of charge carriers of opposite sign from the drain contact. In that regime, both types of charge carriers are present in the device. Thus, the thin-film transistor can be operated in a mixed state in which both electron and hole currents are transported within the device and where the double injection regime is controlled by the gate voltage. Our device exhibits electron and hole mobilities of 3 × 10 −3 cm 2 /Vs and 1 × 10 −4 cm 2 /Vs, respectively. Investigation of a bulk heterostructure of a thienylene derivative and PTCDI-C 13 H 27 results in a light-emitting field-effect transistor. The light emission is controlled by both the drain-source voltage V DS and the gate voltage V G .